Peter Wadhams: Every time I visit the Arctic, the ice gets thinner

In 1971, as a young graduate student, I was immensely excited to be sailing under the sea ice of the Arctic Ocean in a Navy submarine. In those days she was a diesel submarine, HMS Oracle, accompanying the Navy's nuclear sub, HMS Dreadnought, part-way towards the North Pole. The Navy saw that data collected from submarines could be of enormous scientific value, and so began an association which has lasted to the present day.

My fifth voyage in a polar submarine was last year, aboard HMS Tireless, in a survey that took her across the Arctic from Greenland towards the north coast of Alaska. Despite an accident under the ice, which killed two sailors, she accomplished a huge scientific programme including obtaining the first 3D maps of the ice underside with a multibeam sonar.

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As I revisited the underside of the ice every few years, I could see big changes going on. The ice has been getting much thinner – a change of more than 40 per cent in the past 20 years. Sea ice has a rugged underside, with deep pressure ridges pushing blocks of ice down to 50 metres or more. These are disappearing – there are only a quarter as many now as there were 20 years ago.

This makes it much easier for ships to traverse the Arctic; tourists can travel to the Pole in summer aboard icebreakers which in the past were confined to the Arctic's fringes. But most important is the retreat of the Arctic ice.

At first the area dropped by only 3 per cent per decade. But recently it has begun to speed up, as the reduced ice growth in winter allows the ice to break up completely under increased summer warming.

Last summer was a record; only 4 million sq km of ice was left in September compared to the normal 8 million, and a huge area of the central Arctic, which has never been ice-free before, became blue ocean. This year, all the signs are that a greater retreat is in progress, which may for the first time make the North Pole part of an ice-free ocean. This is the biggest signal of climate change that has yet happened, and is visible from space – our blue planet is now blue right up to the North Pole, instead of having a white cap.

Peter Wadhams is professor of ocean physics at Cambridge University

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